In an engine for a vehicle (particularly, internal combustion engine), fuel injected by a fuel injection valve (injector) is ignited and combusted within a combustion chamber in a predetermined cylinder, and torque (power) is generated on a predetermined output shaft (crankshaft). In a diesel engine for a vehicle, before or after a main injection, a sub injection is performed to inject fuel in an amount smaller than that of the main injection, which is called as a multi-step injection. To reduce noise upon fuel combustion and a NOx emission, a pilot injection or pre injection may be performed by a small injection amount before the main injection. Further, for the purpose of activation of diffusive combustion, and reduction of particulate matters (PM), an after injection (performed at injection timing during fuel combustion near the main injection) may be performed after the main injection. Otherwise, for the purpose of activation of a catalyst by the rise of exhaust temperature and/or supply of reduced component, a post injection (performed at injection timing after the completion of combustion much delayed from the main injection) may be performed. Upon engine control in recent years, fuel supply to an engine is performed using one or arbitrary combination of these various injections.
In a system using such multistep injection, because the sub injection is performed by a little amount of fuel, the injection is influenced by environmental condition. For example, in unstable combustion such as PCCI combustion or HCCI combustion, the combustion amount of fuel injected by a pilot injection (pilot combustion amount) is easily changed. The pilot combustion amount fluctuates by e.g. influence of disturbance (fuel property, intake temperature change and the like). When such fluctuation of pilot combustion amount occurs, the combustion characteristic of the fuel injected by a main injection (a main combustion amount, main combustion timing and the like) is also influenced. Accordingly, in multistep injection control, an error (shift) easily occurs in the combustion characteristic of the main combustion. When such error occurs in the combustion characteristic, emission deterioration and unstable combustion status are conceivable.
For example, an apparatus which calculates (detects) main ignition timing (combustion start timing), as ignition timing of main fuel injected by a main injection, by using a cylinder pressure sensor (CPS) to output a detection signal corresponding to pressure in a combustion chamber (cylinder pressure), as disclosed in JP-2004-100559A is proposed. In this apparatus, in a diesel engine (compression-ignition direct-injection engine) as a subject engine, cylinder pressure during engine running is measured by the cylinder pressure sensor, and the main ignition timing is detected based on an output from the sensor, or more particularly, by using correlation between the cylinder pressure and a heat generation rate. The occasional target value can be feedback-controlled to a desired value by variably setting a parameter which acts on the main ignition timing, i.e., a command value of main injection execution timing to a fuel injection valve for direct injection so as to reduce the deviation between a detection value of the main ignition timing and an occasional target value.
FIGS. 17A and 17B show the combustion characteristic (transition of heat generation rate) of a main injection obtained by the inventors by experiment regarding the apparatus disclosed in JP-2004-100559A. Note that FIGS. 17A and 17B are timing charts showing transition of an injection command to the fuel injection valve (pulse signal with a pulsewidth corresponding to injection time) and transition of heat generation rate as a heat amount per unit crank angle (unit output shaft turning angle) generated upon fuel combustion.
As shown in FIGS. 17A and 17B, main ignition timing (the timing can be detected in, e.g., in the waveform of heat generation rate, as timing at which the heat generation rate suddenly changes to the positive side around fuel injection timing) as ignition timing of main fuel injected by a main injection (combustion start timing) is changed in accordance with fuel property (e.g., a cetane number in light oil) More particularly, on a condition where the temperature of a combustion chamber is relatively low such as a low load condition, regarding a low cetane number fuel indicated with an alternate long and two short dashes line L51b in FIGS. 17A and 17B, ignition delay time is longer and the heat generation rate is lower in comparison with a high cetane number fuel indicated with an alternate long and short dash line L51a in FIGS. 17A and 17B.
The time required for fuel ignition in the case of the low cetane number fuel is longer in comparison with the high cetane number fuel, and the combustion rate as the amount of heat per unit fuel amount generated by fuel combustion (corresponding to combustibility) in the case of the high cetane number fuel is higher in comparison with the low cetane number fuel. In the case of the low cetane number fuel, in addition to ignitionability of fuel itself, the amount of heat generated by the pilot injection is smaller than that in the case of the high cetane number fuel, and the ignitionability in the cylinder (strictly, combustion chamber) is lower than that of the high cetane number fuel.
Accordingly, in the case of the low cetane number fuel, the ignition timing of combustion by the main injection (main ignition timing) is timing t50b later than timing t50 in the case of the high cetane number fuel. Further, the time from pilot fuel ignition (timing t50a) to main fuel ignition (timing t50b) (interval between both timings) and the time from execution of main injection (start) to start of combustion (ignition) (main ignition delay time) in the case of the low cetane number fuel are longer than those in the case of the high cetane number fuel. In the case of the low cetane number fuel, as the main ignition delay time is longer, the pressure in the combustion chamber (cylinder pressure) at the main ignition timing and the combustion rate (combustibility) are lowered. Further, in the case of the low cetane number fuel, the maximum heat generation rate regarding the main injection (can be detected as e.g. a maximum point around the above-described main ignition timing in the waveform of heat generation rate) is also lower in comparison with the case of the high cetane number fuel.
In this manner, generally, the lower the cetane number is, the lower the maximum heat generation rate regarding the above-described main injection becomes. Accordingly, when fuel with extremely low cetane number is used, it is impossible to obtain a sufficient combustion amount (heat generation rate) and a sufficient torque. This may cause emission deterioration (white smoke or the like due to increased HC generation amount) and/or degradation of drivability, and at worst, an accidental fire.
According to the apparatus disclosed in JP-2004-100559A, the inventors have performed feedback control of main ignition timing based on variable setting of main injection execution timing (injection start timing), so as to control the main ignition timing in the case of the low cetane number fuel to about the same timing as that in the case of the high cetane number fuel (timing t50 in FIGS. 17A and 17B).
More particularly, a command value related to the above-described main injection execution timing (e.g., the pulse signal indicated in FIG. 17A) is corrected to the timing t52 on the advance side from the uncorrected timing t51 so as to control the detection value of the main ignition timing (the above timing t50b) obtained from the output from the cylinder pressure sensor to the timing t50 on the further advance side. In this arrangement, in the case of the low cetane number fuel, the main ignition timing can be controlled to the timing t50 (in FIGS. 17A and 17B, a solid line L52). Note that regarding correction of the above-described main injection execution timing (strictly, the command value of the main injection execution timing), pilot injection execution timing is also changed (corrected) to timing on the further advance side in correspondence with the change (correction) of the main injection execution timing so as to maintain a constant (or predetermined) interval between both injections.
However, even when the above-described control (feedback control on the main ignition timing) is performed in the case of the low cetane number fuel, the main ignition delay time is longer than that in the case of the high cetane number fuel since the combustion rate cannot be sufficiently increased by the pilot injection. Accordingly, when the above-described main injection execution timing (injection start timing) is advanced too much, a large amount of fuel injected from the start of the main injection, in addition to uncombusted fuel in the pilot injection, is ignited and combusted at once at the main ignition timing (the timing t50 in FIGS. 17A and 17B) as indicated with the solid line L52 in FIGS. 17A and 17B. This may cause excessive heat generation, and excessive rise of the cylinder pressure. When such excessive cylinder pressure rise occurs, noise and/or vibration may occur, otherwise, shock beyond the mechanical strength of the cylinder may be applied to the cylinder, and at worst, may reduce the life of the cylinder or break the cylinder.